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Our legacy fish distribution data contains data by reaches. Reaches are arbitrary locations on streams, eg. reach 1, 2, 3. Each reach's from-to are known, but not by x,y coords. An example is 'brown trout are found in this stream reaches 1 and 2, not 3". Our data has many fish species with such data. I want to create a linear referenced fish distribution database that can dynamically segment this data. It seems I should create a route layer from the NHD for our reaches first, then create event layers for each spp. My question: "Is creating a route layer for reaches necessary? or can I digitize directly from the NHD? I am new to LRS.

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The solution is found in USGS Hydro Event Management(HEM) tools. Using the Import to Events tool, identify the feature class (reaches in this example) to import. This tool creates a two-point line event feature class referenced to the NHD. Since my original reaches feature class geometry is an exact match to NHD streams, it works very clean. Also, the NHD is a route system and does not need further processing. I have not found an equivalent tool in ArcGIS toolbox.

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I think the Linear referencing datasets in ArcGIS page will be your best starting point because it explains (with my bolding) that:

A route feature class is a line feature class that has a defined measurement system. These measurement values can be used to locate events, assets, and conditions along its set of linear features.

You need a route feature class in order to place events at measured distances along it.

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